Effect of austempering time on mechanical properties of a low manganese austempered ductile iron

An investigation was carried out to examine the influence of austempering time on the resultant microstructure and the room-temperature mechanical properties of an unalloyed and low manganese ductile cast iron with initially ferritic as-cast structure. The effect of austempering time on the plane strain fracture toughness of this material was also studied. Compact tension and round cylindrical tensile specimens were prepared from unalloyed ductile cast iron with low manganese content and with a ferritic as-cast (solidified) structure. These specimens were then austempered in the upper (371 °C) and lower (260 °C) bainitic temperature ranges for different time periods, ranging from 30 min. to 4 h. Microstructural features such as type of bainite and the volume fraction of ferrite and austenite and its carbon content were evaluated by X-ray diffraction to examine the influence of microstructure on the mechanical properties and fracture toughness of this material. The results of the present investigation indicate that for this low manganese austempered ductile iron (ADI), upper ausferritic microstructures exhibit higher fracture toughness than lower ausferritic microstructures. Yield and tensile strength of the material was found to increase with an increase in austempering time in a lower bainitic temperature range, whereas in the upper bainitic temperature range, time has no significant effect on the mechanical properties. A retained austenite content between 30 to 35% was found to provide optimum fracture toughness. Fracture toughness was found to increase with the parameter (XγCγ/d)^1/2, where Xγ is the volume fraction of austenite, Cγ is the carbon content of the austenite, and d is the mean free path of dislocation motion in ferrite.     

The influence of cobalt on the austempering reaction in ductile cast iron

Austempering kinetic measurements and mechanical property measurements are reported for two Mn ductile irons with and without Co and three austempering treatments. It is shown that Co accelerates the stage I reaction in each of the irons and for each of the austempering treatments but has little affect on the stage II reaction. Consequently, the processing window is widened and moves to earlier austempering times. This can be useful in the austempering of thicker section components to obtain the higher ductility grades of the ADI standard and to increase process productivity.     

低碳球铁的等温淬火工艺研究

对低碳球铁的等温淬火工艺进行了研究.测定了低碳球铁件在几种不同奥氏体化时间、等温温度、等温时间下的力学性能,并对金相组织进行了分析.实验结果表明:在900℃奥氏体化40min,340℃进行30min的盐浴处理,能得到σb=1040MPa,δ=3%,ak=42J/cm2,HRC=32的较高的力学性能.     

Effect of nodule count and austempering heat treatment on segregation behavior of alloying elements in ductile cast iron

The equilibrium partition ratio, k, has been measured for Mn, Mo, Si, Ni and Cu in a ductile iron with composition(wt.%): 3.45 C, 0.25 Mn, 0.25 Mo, 2.45 Si, 0.5Ni and 0.5Cu with different nodule counts obtained from different section sizes of13, 25, 75 mm in the as cast, austenitized(at 870 °C for times 1, 4 and 6 hours) and austempered(at 375 °C for times 1 to 1,440 min) samples. Results show that Mn and Mo segregate positively at cell boundaries, but Si, Ni and Cu concentrate in an inverse manner in the vicinity ofgraphite nodules and there is a depletion ofthese elements at cell boundaries. Segregation curves for Ni and Cu are more smooth than for Si. Carbide formation has been observed at cell boundaries. Based on the results, the partition ratios for all elements decrease with increasing the nodule count. More carbide with coarser morphology has been observed in the microstructure with a lower nodule count. Austenitization for a longer time can decrease partition ratio, but cannot eliminate it entirely. Increasing the austenitization temperature has the same effect. Austenitizing parameters have no significant effect on carbides volume fraction. The kinetics ofaustempering is faster in higher nodule counts and subsequently better mechanical properties including higher ductility, strength and toughness have been observed for all austempering conditions studied.     

The use of resistivity to investigate the effect of austenitising temperature on austempering of ductile iron in the upper ausferrite region

The resistivity of a ductile iron with 0.77% Cu, 0.5% Ni during austempering at 400 °C was measured to investigate the influence of austenitising temperature on the kinetics of phase transformation. The specimens were heat-treated in a vacuum furnace by passing through a direct current which was monitored and controlled with a personal computer to follow the heating and cooling scheme. Three temperatures, 950, 900 and 850 °C were used to austenitise the ductile iron matrix. After a holding period at the austenitising temperature, the specimens were quenched, with a liquid nitrogen spray, to 400 °C to conduct the isothermal transformation. During the isothermal transformation at 400 °C, the resistivity varied with time and clearly showed three sequences of transformation: stage 1; the process window; and stage 2; The changes of resistivity around the process window were related to the volume fraction of retained austenite which was measured by the X- ray diffraction technique. A model is then proposed to predict the volume fraction of retained austenite from the change of resistivity for austempering during the period of the process window. A further kinetics analysis demonstrates that the reaction of all three stages obey the Johnson-Mehl equation with average values of exponent n equal to 1.25, 1.07, and 1.48 respectively. It is also shown that increasing the austenitising temperature will reduce the reaction rate and suppress the start of the austempering reaction.     

关于奥贝球铁及其微观组织术语的探讨—对ADI(AustemperedDuctileIron)我们需要一个正确的中文术语

论述了等温淬火球铁的工艺及其微观组织。分析了其微观组织与钢中贝氏体的区别。指出ADI（Austempered Ductile Iron）奥贝球铁是一个恰当的术语。对这种新铸铁,我们需要一个正确的中文术语：奥铁球铁或者称奥氏体等温淬火球铁。     

等温淬火球墨铸铁在商用车轮毂上的应用实例

介绍了QTD800-10汽车轮毂的研制过程。主要采用高纯度生铁、低锰废钢熔炼铁液,并进行喂丝球化和多次孕育,控制化学成分(质量分数)为:C 3.70%、Si 2.60%、Mn 0.30%、P 0.025%、S≤0.02%、RE 0.015%,Mg 0.045%、Cu 0.70%。对浇注后的轮毂进行840℃、120 min奥氏体化+350℃、60 min的等温淬火,所得毂抗拉强度达845 MPa、伸长率为12.4%、硬度为260 HBW,性能指标完全达到客户要求,同时实现了产品轻量化。     

等温淬火球墨铸铁的滑动磨损

研究了奥贝球铁、下贝球铁及与其基体组织相同的钢的滑动磨损性能。实验结果表明 :在一定条件下 ,石墨对等温淬火球铁耐磨性无补而有损。由于奥贝球铁的转折载荷随摩擦速度的提高而增大 ,当转速为 980r/min ,载荷 >6 8 6N时 ,等温淬火球铁的耐磨性优于钢     

等温淬火球铁及其在国内开发应用前景展望

由于等温淬火球墨铸铁(ADI)具有优异的综合力学性能，已在工业发达国家得到了广泛应用。本文概述了ADI的等温淬火工艺和影响ADI性能的因素。在此基础上，针对国内的现实情况，展望了ADI在我国的发展趋势。     

The influence of cast structure on the austempering of ductile iron. Part 3. The role of nodule count on the kinetics,microstructure and mechanical properties of austempered Mn alloyed ductile iron

Austempering kinetic measurements and mechanical property measurements are reported for irons with different Mn contents and different nodule counts after austenitising at 870 °C and austempering at 375 °C. It is shown that increasing nodule count, which reduces segregation and changes the size and distribution of intercellular boundaries, increases the interphase boundary area between graphite and matrix and decreases the continuity of the unreacted austenite in the intercellular boundary. This accelerates the stage I reaction which broadens the heat treatment window and moves it to earlier austempering times. A high nodule count can be used to counter the delay of the stage I reaction caused by Mn additions used to increase the hardenability of the iron. A high nodule count produces a finer, more uniform ausferrite structure that increases the strength, ductility and impact energy of the austempered iron.     

等温淬火温度对奥贝球铁水脆化行为的影响

研究了等温淬火温度对奥贝球铁(ADI)水脆化行为的影响，水附着条件下不同等温淬火温度处理的ADI均发生水脆化行为，抗拉强度和伸长率显著降低；但随着等温淬火温度升高，ADI的水脆化程度降低。高强度的ADI、淬火回火球铁和珠光体球铁均发生水脆化行为，而铁素体含量高的铸态球铁和铁素体球铁无明显的水脆化行为。     

等温淬火对含铬球墨铸铁组织及性能的影响

研究了等温淬火工艺对含铬球墨铸铁组织、硬度、冲击性能和耐磨性的影响。结果表明：奥氏体化温度升高,能促进球状石墨长大,增加残留奥氏体含量。淬火后组织主要为球状石墨、针状贝氏体、含铬碳化物及残留奥氏体。当淬火等温温度在240～270 ℃,随着等温温度升高,试样硬度和耐磨性均降低;在240 ℃等温时冲击韧度较低,继续升高等温温度,冲击韧度先增大后降低;当试样经910 ℃×80 min奥氏体化、270 ℃×180 min等温淬火后,含铬球墨铸铁的硬度可达54.1 HRC、冲击韧度αk可达8.1 J·cm-2,有较好的耐磨性。     

Austempering and austempered ductile iron microstructure in copper alloyed ductile iron

The variation in the austempered microstructure, the volume fraction of retained austenite, X_λ, the average carbon content of retained austenite, C_λ, their product X_λC_λ and the size of bainitic ferrite needles with austempering temperature for 0.6% Cu alloyed ductile iron have been investigated for three austempering temperatures of 270, 330, and 380 °C for 60 min at each temperature after austenitization at 850 °C for 120 min. The austempering temperature not only affects the morphology of bainitic ferrite but also that of retained austenite. There is an increase in the amount of retained austenite, its carbon content, and size of bainitic ferrite needles with the rise in austempering temperature. The influence of austempering time on the structure has been studied on the samples austempered at 330 °C. The increase in the austempering time increases the amount of retained austenite and its carbon content, which ultimately reaches a plateau.     

A study on controlled cooling process for making bainitic ductile iron

In the present research, TTT curve of bainitic ductile iron under the condition of controlled cooling was generated. The cooling rate of grinding ball and its temperature distribution were also measured at the same time. It can be concluded that the bainitic zone of TTT curve is separated from the pearlitic zone. As compared to the water-quenching condition, more even cooling rate and temperature distribution can be achieved in the controlled cooling process. The controlled cooling can keep away from pearlitic zone in the high temperature cooling stage and produce similar results to the process of traditional isothermal cooling with a low cooling rate in the low temperature cooling stage.     

Effect of low temperatures on charpy impact toughness of austempered ductile irons

Impact properties of standard American Society for Testing Materials (ASTM) grades of austempered ductile iron (ADI) were evaluated at subzero temperatures in unnotched and V-notched conditions and compared with ferritic and pearlitic grades of ductile irons (DIs). It was determined that there is a decrease in impact toughness for all ADI grades when there is a decrease in content of retained austenite and a decrease in test temperature, from room temperature (RT) to −60 °C. However, the difference in impact toughness values was not so noticeable for low retained austenite containing grade 5 ADI at both room and subzero temperatures as it was for ADI grade 1. Furthermore, the difference in impact toughness values of V-notched specimens of ADI grades 1 and 5 tested at −40 °C was minimal. The impact behaviors of ADI grade 5 and ferritic DI were found to be more stable than those of ADI grades 1, 2, 3, and 4 and pearlitic DI when the testing temperature was decreased. The impact toughness of ferritic DI was higher than that of ADI grades 1 and 2 at both −40 °C and −60 °C. The impact properties of ADI grades 4 and 5 were found to be higher than that of pearlitic DI at both −40 °C and −60 °C. The scanning electron microscopy (SEM) study of fracture surfaces revealed mixed ductile and quasicleavage rupture morphology types in all ADI samples tested at both −40 °C and −60 °C. With decreasing content of retained austenite and ductility, the number of quasicleavage facets increased from ADI grade 1–5. It was also found that fracture morphology of ADI did not experience significant changes when the testing temperature decreased. Evaluation of the bending angle was used to support impact-testing data. Designers and users of ADI castings may use the data developed in this research as a reference.     

Effect of austenitizing conditions on the impact properties of an alloyed austempered ductile iron of initially ferritic matrix structure

The effect of austenitizing conditions on the microstructure and impact properties of an austempered ductile iron (ADI) containing 1.6% Cu and 1.6% Ni as the main alloying elements was investigated. Impact tests were carried out on samples of initially ferritic matrix structure and which had been first austenitized at 850,900, 950, and 1000°C for 15 to 360 min and austempered at 360°C for 180 min. Results showed that the austenitizing temperature, T_γ, and time, t_γ, have a significant effect on the impact properties of the alloy. This has been attributed to the influence of these variables on the carbon kinetics. The impact energy is generally high after short t_γ, and it falls with further soaking. In samples austenitized at 850 and 900°C, these trends correspond to the gradual disappearance of the pro-eutectoid ferrite and the attainment of fully developed ausferritic structures. In initially ferritic structures, the carbon diffusion distances involved during austenitization are large compared to those in pearlitic structures. This explains the relatively long soaking periods required to attain fully ausferritic structures, which in spite of the lower impact energy values, have a better combination of mechanical properties. Microstructures of samples austenitized at 950 and 1000°C contain no pro-eutectoid ferrite. The impact properties of the former structures are independent of t_γ, while those solution treated at 1000°C are generally low and show wide variation over the range of soaking time investigated. For fully ausferritic structures, impact properties fall with an increase in T_γ. This is particularly evident at 1000°C. As the T_γ increases, the amount of carbon dissolved in the original austenite increases. This slows down the rate of austenite transformation and results in coarser structures with lower mechanical properties. Optimum impact properties are obtained following austenitizing between 900 and 950°C for 120 to 180 min.     

Polyurethane coating for ductile iron pipes

Polyurethane coating has been widely used in pipe protection because of its perfectly mechano-chemistry properties and convenient application. But, being a solvent coating, a thick coat can not be obtained by spraying once with a common polyurethane coating. Moreover, the common polyurethane coating cures slowly as a result of being a secondary hydroxy polyether polyol. Therefore, it does not meet the requirements of rapid current production nor is it suitable for spraying on the surface of du…     

一种球墨铸铁等温正火工艺

对一种球墨铸铁铸件进行了等温正火处理,分析了其不同等温正火工艺下的布氏硬度和显微组织。结果表明,等温温度一定时,随着正火加热温度的升高,硬度随之升高,当正火加热温度一定时,随着等温温度提高,硬度逐渐降低。加热温度为900 ℃、等温温度为650 ℃处理后球铁铸件的显微组织为大量珠光体（>85%）和少量铁素体,硬度较高（375~380 HB）;正火温度较低（850 ℃）、等温温度较高（700 ℃）时的显微组织为大量的游离铁素体（>60%）和少量珠光体,硬度较低（200~205 HB）。根据试验结果提出了一种较为合理的球墨铸铁等温正火工艺。     

奥贝球铁齿轮在柴油机上的应用研究

研究了不同等温温度及Mn、Si、Cu、Mo对奥贝球铁组织和力学性能的影响,讨论了奥贝球铁加工硬化对机加工性能的影响。同时,奥贝球铁齿轮代替40Cr调质钢齿轮进行装机试验,对降噪声和耐磨性进行了测试。结果表明,奥贝球铁齿轮与40Cr调质钢齿轮相比,力学性能高,柴油机整机噪声下降1.92dB,齿轮侧噪声下降5.3dB,奥贝球铁齿轮经过45h磨合后,在标定工况下连续运转200h,齿轮无异常磨损,齿轮间隙离极限间隙还有0.13mm的余量,所以其摩擦磨损性能合格,所生产的奥贝球铁齿轮满足设计使用要求。     

低合金奥-贝球铁电弧焊焊条的研究

本文通过等温热处理系统地研究了Ｃｕ，Ｎｉ，Ｍｏ对球铁焊缝金属等温转变，奥－贝化能力及机械性能的影响，并利用Ｘ射线衍射仪，扫描电镜，透射电镜研究了低合金奥－贝球铁焊缝的基体结构，分析讨论了基强韧化机制。在此基础上首次研制成功了Ｎｉ－Ｍｏ低合金奥－贝球铁新焊条，与非合金化奥－贝球铁焊条相比，焊缝金属的奥－贝化能力提高２．５倍，其焊缝与焊接接头的机械性能均满足奥－贝球铁的要求。